The first thing to choose when designing a power supply are the batteries, since they take up most of the room and weight. When choosing batteries one must think about size, weight, capability. But also availability and mounting to the PCB. Also non-hardcase batteries tend to swell in vacuum and you can't trust batteries with built in electronics like cell batteries in space either.

Electrical power system with two 18650 battery.

First option is 18650 lithium ion cells. Standard practice, easy to get hold off and you can get space certified versions. But they are really uncomfortable package for PCB stacking. For a minute I though about AAA sized liion cells, one can get thinner power system, when using them. But they are as hard to assemble as 18650 cells and there has to be quite many of them (10-12 pieces).

Using AAA sized lithium ion cells for storage.

Another option is to use RC hobby market to get cheap and good batteries. Like these 5.6Ah lithium polymer cells - cheap to buy, easy to assemble, no worries about blowing them up. Unfortunately lithium polymer has two times less energy density than lithium ion. Normally we don't have high current busts in common satellite power system so lithium polymer battery strength will be unused.

After searching around for a long time I finally found some pages that sold prismatic lithium ion cells. One idea is to use two 1.5 Ah soft case cells and press them between pcbs. That would get us 11.1 Wh of capacity with 10 mm thick power system.

EPS using prismatic lithium ion cells.

Finally I found Panasonic UF103450P hard case prismatic lithium ion cells. They are flat and look like phone batteries and since they are hard case - we can glue/solder them on the PCB and be done with it. Sourcing will be a bit hard, but the result - 13.3 Wh system at 12 mm thick, single PCB power system is really good. After a bit of search I found multiple companies that sell prismatic lithium ion cells. Like this and this. The second site (a bit down at the moment) gives us 6.7 mm 2Ah batteries. Excellent!

All of the final battery options are good and worth using in a satellite, which one to use depends on the client (you!).